JP3610883B2 - Method for producing high-tensile steel sheet with excellent bendability - Google Patents

Description

表１で鋼ＨおよびＩはＣ含有量が高い比較例であり、他は本発明の規定する条件を満足する化学組成を有するものである。これらの鋼塊を加熱炉に装入し、１２００℃で６０分間保持した後、炉から取り出し、粗圧延して厚さ：３０ｍｍの鋼片とした。
【００５５】
これらの鋼片に圧延開始温度を１０００℃とする熱間仕上圧延を施した。一部の鋼片は仕上圧延開始前に加熱炉に装入し、５０℃程度昇温する補助加熱を施して仕上圧延開始温度を１０３０℃とした。仕上圧延のパス回数は合計６パス、仕上圧延後の厚さは４．０ｍｍであり、仕上圧延終了温度は８５０℃〜７５０℃の間で種々変更した。
【００５６】
熱間仕上圧延終了後、５〜４０℃／秒の範囲の種々の冷却速度で冷却し、巻取温度を模擬するために、７００〜６００℃の間の種々の温度の熱処理炉に装入し、３０分間保持した後、２０℃／時で室温まで冷却した。得られた熱延板は、塩酸溶液を用いて酸洗してスケールを除去した後、合計圧下率６０％で１．６ｍｍ（２例のみ１．２ｍｍ、合計圧下率７０％）に冷間圧延した。
【００５７】
得られた冷延板を、種々の条件で焼鈍した後、５〜２０℃／秒の冷却速度で常温まで冷却した。焼鈍は２相域焼鈍のみを施したものを主としたが、後述する試験番号３および９では、γ域温度で焼鈍した後に２相域焼鈍を行った。
【００５８】
引張特性：得られた冷間圧延鋼板から、長手方向を圧延方向（Ｌ）、幅方向（Ｔ）、または、圧延方向から４５°傾斜した方向（Ｃ）とする、ＪＩＳに規定された５号試験片を採取して引張試験をおこない、引張強さ（ＴＳ）と全伸び（Ｅｌ）について、３方向の平均値を求めた。
【００５９】
曲げ性：得られた冷間圧延鋼板から、上記と同様に、長手方向を上記３方向とする曲げ試験片を採取して密着曲げ試験をおこない、曲げ部外側を拡大鏡で拡大して目視観察し、その表面の割れ、または、くびれの発生状況を以下の基準で判定し、○を合格とした。くびれは、ネッキング傾向が観察された試験片の曲げ部の曲げ軸に対して垂直な断面を切り出し、研磨して顕微鏡観察をおこない、試験前の板厚に対する板厚減少率が２０％以上であるものを「くびれ有り」と判断した。
○：割れ無し、くびれ無し、
△：割れ無し、くびれ有り、
×：割れ発生。
【００６０】
表２に、製造条件と特性値測定結果をまとめて示す。
【００６１】
【表２】

表２で、ＦＦＴ は熱延仕上圧延終了温度、ＣＲ１ は熱延後冷却速度、ＣＴは巻取温度、ＲＡ１ はγ域焼鈍温度、ＲＡ２ は２相域焼鈍温度、ＣＲ２ は２相域焼鈍後の冷却速度、ＴＳは引張強さの３方向平均、Ｅｌは全伸びの３方向平均値、を意味する。
【００６２】
表２に示すように、試験番号１、２、３、６、８、９、１１、１３および１４では、３方向共密着曲げ試験において割れもくびれも発生しておらず、良好な曲げ性を有するものであった。これに対し、試験番号４、５、１０、１２、および１５〜１８では、いずれかの曲げ方向において割れまたはくびれが発生した。
【００６３】
（実施例２）
図１は、縦壁部を折り曲げ、密着曲げして重ね合わせたチャンネル形状部品の斜視図である。図１で、符号Ｂは縦壁折り曲げ部であり、縦壁部はＡ部で密着曲げされる。図１でａは２５０ｍｍ、ｂは８０ｍｍ、ｃは１００ｍｍであった。
【００６４】
実施例１の試験番号１および１８の鋼板の板面内で、長さ方向をＬ方向またはＴ方向とする、上記チャンネル形状部品用のブランクを切り出し、図１に示すチャンネル形状部品に成形した。試験番号１の鋼板では、ブランクの採取方向をいずれの方向にしても密着曲げ部（Ａ）には割れやくびれがなく良好に加工された。試験番号１８の鋼板では、チャンネル形状部品の長手方向をＬ方向にして採取したブランクで密着曲げ部（Ａ）に割れが発生した。
【００６５】
このことは、上記チャンネル形状部品の成形において、その長さ（ａ）が鋼板幅を超える場合には、試験番号１８の鋼板では加工困難であることを意味する。従って試験番号１８の鋼板により、その幅を超える寸法の上記形状の部品を得るには、密着曲げ加工の無い方法に依るか、複数部品を長手方向に継ぎ足すなど、非効率な方法に依らざるを得ない。
【００６６】
【発明の効果】
本発明の高張力鋼板は、板面内いずれも方向に対しても優れた密着曲げ性を有するので、盤面内での方向に制約されることなく自由に曲げ成形用素材を得ることができる。従って素材の幅に制約されることなく、歩留まりの良い方法で良好な成型品を得ることができる。従って本発明の高張力鋼板およびその製造方法は、高張力鋼板の適用拡大に大きく寄与し。工業上の利用価値が極めて大きい。
【図面の簡単な説明】
【図１】縦壁部を折り曲げ、密着曲げして重ね合わせたチャンネル形状部品の斜視図である。種々の方向の曲げ部を有するモデル部品の外観図である。
【符号の説明】[0001]
The present invention relates to a high-tensile steel sheet having excellent bendability and a method for producing the same. More specifically, a cold-rolled high-tensile steel with a two-phase structure consisting of ferrite and martensite, a tensile strength of 500 MPa or more, and capable of tight bending in all directions.PlankIt relates to a manufacturing method.
[0002]
[Prior art]
In recent years, the weight reduction of a vehicle body is progressing for the improvement of the fuel consumption of a motor vehicle, and the application of the high-tensile steel plate to the vehicle body structural member is advanced. The weight reduction effect is enhanced as the strength of the steel sheet used is higher. For this reason, for example, high tensile strength steel plates having a tensile strength of 600 MPa or more have been applied to vehicle body structural member applications such as frames. However, in general, the strength and formability of a steel sheet are in a contradictory relationship, and as the strength of the steel sheet increases, the formability decreases, and there is a problem that application of a high-strength steel sheet becomes difficult.
[0003]
For this reason, a high-tensile steel plate excellent in formability has been demanded, and various steel plates have been developed in response thereto.
For example, Japanese Patent Application Laid-Open No. Sho 54-163718 contains C: 0.03 to 0.12% by mass and Mn: 0.7 to 1.7%, and mainly includes a ferrite phase and a quenched transformation phase. A method for producing a composite structure steel sheet as a structural component and having a yield ratio (yield strength / tensile strength) of 0.6 or less is disclosed.
[0004]
JP-A-9-3594 includes C: 0.02 to 0.20%, Si: 0.20 to 0.8%, and Mn: 0.30 to 2.5% by mass. A high-strength steel sheet that satisfies specific Ceq, is excellent in uniform elongation and is composed of ferrite and martensite as the second phase, and a method for producing the same are disclosed.
[0005]
These steels are steels having a composite structure in which hard martensite is dispersed in a ferrite structure (dual phase steel sheet; hereinafter referred to as “DP steel sheet”). By adjusting the martensite ratio, It is easy to increase the tensile strength of steel to the desired range, and it can also be characterized as having a low yield ratio and excellent uniform elongation, which is expected as a high-tensile steel sheet for forming. I came.
[0006]
[Problems to be solved by the invention]
In bending forming, which is the basic forming method among various forming methods, in soft bending steel and high strength steel plate with low strength whose tensile strength is less than 500 MPa, close bending (inner bending radius is zero, bending angle is 180 °) is easy. However, when the tensile strength is in a high strength region of 500 MPa or more, even DP steel plates are in close contact in bending in the rolling direction (bending in which the bending axis is perpendicular to the rolling direction), which is considered to have excellent bending performance. Bending may be difficult. Bending in the width direction (bending in which the bending axis is parallel to the rolling direction), or 45 ° bending (bending axis is inclined 45 ° with respect to the rolling direction), which is considered to be inferior to the bending in the rolling direction. In many cases, the close contact bending in a certain bend becomes more difficult. Such a difference in bendability depending on the bending direction (hereinafter, the difference in bendability in the three directions is also referred to as “bending anisotropy”) becomes more significant as the tensile strength of the steel plate increases.
[0007]
The DP steel sheet is characterized by a low yield ratio and excellent uniform elongation due to the mixing effect of soft ferrite and hard martensite, and therefore has good shape accuracy in press forming and excellent stretch formability. However, the local ductility is not excellent and the bendability is not sufficient, and the tight bending becomes difficult as the tensile strength becomes higher, the bending anisotropy becomes stronger, and the bendability is not sufficient depending on the bending direction. there were.
[0008]
For this reason, for example, in bending forming when manufacturing a frame or the like, DP steel plates that are currently used cannot obtain good processing results depending on the forming direction of the material, resulting in a decrease in yield, an increase in processing man-hours, and material processing. There were inconveniences such as limitation of direction.
[0009]
The object of the present invention is to solve these problems, and in a high-strength DP steel sheet, it has excellent bending workability in all bending directions and has high bending strength and excellent bending properties.PlankIt is to provide a manufacturing method.
[0010]
[Means for Solving the Problems]
The present inventor made DP steel sheets with various chemical compositions and production conditions, and conducted various studies on factors affecting the bendability of DP steel sheets and their improvement methods. As a result, after heating and rough rolling a steel having a specific chemical composition, starting at 1050 ° C. or less, and performing hot finish rolling completed at Ar 3 point to Ar 3 point + 100 ° C., then 20 ° C./second or less The sheet is cooled at a cooling rate of 600 ° C., wound at 600 ° C. or higher, pickled, cold-rolled at a reduction rate of 50 to 70%, annealed in the (α + γ) two-phase region for 30 to 90 seconds, and up to 550 ° C. It was found that a DP steel sheet having good adhesion bending was obtained in all of the bending in the rolling direction, the bending in the width direction, and the bending in the 45 ° direction by cooling at a temperature of ° C / second or more.
[0011]
In the present invention, the criterion for close contact bending is defined as good when cracks and constriction (necking) are not observed on the outer surface of the bent part subjected to close contact bending. In the present invention, the above “necking” is accompanied by a local thickness reduction of 20% or more with respect to the surrounding thickness.
[0012]
The reason is not clear, but is presumed as follows. In bending, deformation occurs in which a strain in a direction perpendicular to the bending line is pulled outside the bend and compressed inside. It is considered that cracking occurs at a portion in the outer tensile stress state at the time of bending due to a deformation process similar to a constriction deformation that occurs after uniform elongation deformation in a normal tensile test, that is, a fracture after local deformation.
[0013]
It seems that cracks in bending work are caused by insufficient local ductility in the tensile test. In particular, when the ferrite and second phase of the steel sheet are layered and have a crystal structure that extends in the rolling direction (hereinafter referred to as “band structure”), cracks occur at the interface between the band structure and the ferrite phase. Since it is easy to do, the local ductility in the part falls and bendability is impaired greatly.
[0014]
The band-like structure is likely to occur when the finish rolling when the cold-rolled base metal is hot-rolled becomes (α + γ) two-phase region rolling, and when pearlite is generated during cooling after finish rolling, this is a band-like structure. Easy to form organization. Therefore, the cold rolling base material is preferably manufactured under conditions that allow a hot rolled steel sheet having a crystal structure composed of ferrite and cementite to be obtained. It is also preferable to perform γ region annealing prior to two-phase region annealing after cold rolling.
[0015]
In order to realize the above-described conditions in hot rolling finish rolling, it is necessary to strictly control the steel plate temperature in hot rolling. As this means, auxiliary heating means such as a coarse bar heater can be used to measure the coarse bar before finish rolling. It is extremely effective to control the temperature fluctuation with high accuracy. For the same purpose, it is also effective to apply a so-called coil box for heat insulation or heating, or to join a rough bar and perform continuous rolling.
[0016]
The present invention has been completed on the basis of these newly obtained findings, and the gist thereof is as follows (1) to(7)High-strength steel with excellent bendability as described inPlankIn the manufacturing method.
[0017]
(1) By mass%, C: 0.05 to 0.10%, Si: 1.0% or less, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.01 % Or less, Al: 0.005 to 0.10%, N: 0.05% or lessThe balance is Fe and inevitable impuritiesChemical compositionIs heated at a temperature of 1050 ° C. or lower, finished at Ar 3 points or higher and Ar 3 points + 100 ° C. or lower, cooled at a cooling rate of 20 ° C./second or lower, and 600 ° C. or higher. Is rolled into a hot rolled sheet, pickled and cold-rolled at a rolling reduction of 40 to 80%, and held in a two-phase region of Ac1 point or more and less than Ac3 point for 30 to 90 seconds. A method for producing a high-tensile cold-rolled steel sheet excellent in bendability, characterized by cooling to 550 ° C. at a cooling rate of 5 ° C./second or more after performing two-phase region annealing.
[0018]
(2) Furthermore, the chemical composition contains 0.003 to 0.05% in total of one or more of the group consisting of Ti, Nb, and V in terms of mass% (1 ) High tensile strength with excellent bendabilityCold rollingsteel sheetManufacturing method.
[0019]
(3) Further, as a chemical composition, Cr (0.001 to 2.0%) and / or Mo (0.001 to 0.5%) are contained by mass%, (1) or High tensile strength with excellent bendability as described in (2)Cold rollingsteel sheetManufacturing method.
[0020]
(4) Further, as a chemical composition, in the group consisting of Ca: 0.0002 to 0.004%, Zr: 0.005 to 0.05%, and rare earth elements: 0.002 to 0.05%. High tension excellent in bendability according to any one of the above (1) to (3), characterized by containing one or more ofCold rollingsteel sheetManufacturing method.
[0022]
(Five) Prior to the two-phase region annealing, the γ region annealing is performed for 10 to 90 seconds at a γ region temperature of Ac3 point or higher and Ac3 point + 80 ° C or lower.(1) ~ (Four) EitherHigh tension with excellent bendability as described inCold rollingA method of manufacturing a steel sheet.
[0023]
(6) Auxiliary heating is applied to the steel slab before finishing rolling.(1) ~ (Five) EitherHigh tension with excellent bendability as described inCold rollingA method of manufacturing a steel sheet.
(7) the above (1) ~ (6) A method for producing a high-tensile hot-dip galvanized steel sheet, further comprising subjecting the high-tensile cold-rolled steel sheet obtained by the production method according to any one of the methods to hot-dip plating.
In addition, the high-tensile steel plate of the present invention is a steel plate that has been subjected to cold rolling and recrystallization annealing after hot rolling under the above conditions, and can be a hot-dip galvanized steel plate that has been subsequently galvanized.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in more detail.
Chemical composition;
C: C has an effect of improving the strength of steel at a low cost, and in order to secure a desired strength, 0.05% or more of C is contained. The content may be changed depending on the target strength. If C is excessively contained, spot weldability is impaired, so the C content is 0.10% or less. Preferably it is 0.08% or less
Si: Although not an essential element, it has an action of deoxidizing steel, and is an element effective for improving the strength of steel without significantly impairing ductility. On the other hand, if too much Si is contained, the plating property is impaired. Therefore, in order to acquire the said effect, you may make it contain in 0.01% or more and 1.0% or less, Preferably it is 0.8% or less.
[0025]
Mn: An effect of increasing the tensile strength of the steel sheet and preventing hot brittleness, and is an important element in the steel of the present invention. In the present invention, as described above, in order to limit the contents of C and Si, which are strength improving elements, to a relatively low range, the tensile strength of the steel sheet is ensured mainly by containing Mn. In order to exert this effect, the Mn content is 1.0% or more. Preferably it is 2.0% or more. Mn is an expensive element, and if the content is excessively increased, melting and refining of the steel in the converter becomes difficult. In order to avoid this, the Mn content is 3.0% or less.
[0026]
P: An unavoidable impurity, but when the P content exceeds 0.05%, segregation in the steel sheet becomes significant, and a band-like structure is formed in the product steel sheet, which deteriorates bendability. In order to avoid this, the P content is 0.05% or less.
[0027]
S: An unavoidable impurity, but when the S content exceeds 0.01%, the amount of precipitation as MnS increases, which impairs the ductility of the steel sheet. In order to avoid this, the S content is set to 0.01% or less.
[0028]
Al: As a deoxidizer for obtaining sound steel, 0.005% or more is contained. Preferably it is 0.01% or more. If the Al content exceeds 0.10%, the inclusions in the steel increase and the elongation characteristics deteriorate, so the upper limit is made 0.10%. Preferably it is 0.05% or less. Note that Al in the present invention is so-called sol. Means Al.
[0029]
N: An unavoidable impurity, and its content is preferably low. In particular, when the N content exceeds 0.05%, the amount of Al consumed as AlN is large and not only the effect of Al addition becomes small, but also the AlN as a precipitate increases and the ductility is impaired, so the N content Is 0.05% or less. Preferably it is 0.01% or less.
[0030]
The steel plate constituting the present invention is basically composed of the above-mentioned composition, but when the strength and formability of the steel plate are further required, the elements described below may be contained.
Ti, Nb, V: Although not essential elements, these elements have precipitation strengthening action, and are elements useful for strengthening steel at low cost. Therefore, they may be contained in order to obtain the effect. . In order to acquire the said effect, it is good to contain 1 type or 2 types or more in the group which consists of Ti, Nb, and V in total 0.003% or more. If the content exceeds 0.05%, the effect of strengthening the steel is saturated, and adding more than this impairs economic efficiency. Therefore, the upper limit in the case of inclusion is 0.05% or less in total.
[0031]
Cr, Mo: Since these have a transformation strengthening action, they may be contained in order to strengthen the steel. In order to acquire the said effect, it is good to make it contain 0.001% or more. However, since the effect is saturated even if Cr is contained in excess of 2.0% and Mo is contained in excess of 0.5%, the upper limit in the case of inclusion is the above value.
[0032]
Ca, Zr, rare earth elements: Since these elements are effective elements for controlling the form of inclusions, they may be contained in order to further improve the bendability. In order to obtain a desired effect, it is preferable to contain 0.0002% for Ca, 0.005% for Zr, and 0.002% or more for rare earth elements. However, if Ca is contained in an amount exceeding 0.004%, Zr in the case of 0.05%, and rare earth elements in an amount exceeding 0.05%, inclusions in the steel increase and workability is impaired. . Therefore, when these elements are used, it is preferable that the content is 0.004% or less for Ca, 0.05% or less for Zr, and 0.05% or less for rare earth elements. The inclusion morphology can be controlled by using any one of these elements, but two or more may be added in combination.
[0033]
Components other than the above are Fe and inevitable impurities. For example, even if Cu, Co, etc. are contained in a small amount, the intended effect of the present invention is exhibited.
When the steel plate strength is less than 500 MPa, a steel plate that can be tightly bent in any direction with respect to the rolling direction can be easily obtained. Therefore, the steel sheet which is the subject of the present invention has a tensile strength of 500 MPa or more. The upper limit of the tensile strength is not particularly limited, but the ductility decreases as the tensile strength increases, and press working by a normal method becomes difficult. Therefore, in the present invention, the tensile strength is 1000 MPa or less. It is good to do. More preferably, it is 850 MPa or less.
[0034]
The high-tensile steel sheet of the present invention has a two-phase structure in which martensite is 5% or more and 50% or less in volume ratio, and the balance is substantially made of ferrite in order to secure properties such as tensile strength and low yield ratio of the steel. It shall have. When the martensite ratio is less than 5% by volume, performance such as desired strength and low yield ratio (for example, yield ratio is 0.65 or less) cannot be obtained. Preferably it is 10 volume% or more. On the other hand, if the martensite ratio exceeds 50% by volume, the workability is excessively lowered. Preferably it is 30 volume% or less.
[0035]
The meaning of “the balance is substantially composed of ferrite” means that even when a structure such as bainite or residual austenite is mixed in an amount of 10% by volume or less during martensitic transformation in the cooling process after cold rolling and annealing. It means that there is no problem in exerting the effect of the invention.
[0036]
The steel sheet of the present invention has excellent bendability characterized by being capable of tight bending in any direction within the plate surface in order to improve the press forming efficiency of the high-tensile steel sheet. The change in bendability depending on the bending direction within the plate surface can be confirmed by the three-direction bendability represented by the bending in the rolling direction, the bending in the width direction, and the bending in the 45 ° direction. “Close-contact bending is possible in any direction in the plane” means that close-contact bending is possible in the above three directions. Criteria for close contact bending are good when cracks and constrictions (with a local thickness decrease of 20% or more with respect to the surrounding plate thickness) are not observed on the outer surface of the bent portion subjected to close contact bending. It is defined as
[0037]
With such bendability, when processing frames, etc., good processed products can be obtained regardless of the direction of close-contact bending in the steel plate surface, improving product yield and processing. Various industrially significant benefits such as a reduction in man-hours or a reduction in component manufacturing cost due to an increase in the degree of freedom of plate cutting can be obtained.
[0038]
Production method;
The high-strength steel sheet excellent in bendability of the present invention is obtained by hot rolling, cold rolling, and recrystallizing steel having the chemical composition described in any of the above (1) to (4) by the following method. It is preferable to manufacture by annealing.
[0039]
Steel having the above chemical composition is cast into a slab by a conventional method. The ingot may be cracked and the steel slab may be used as a slab. The slab is heated by a conventional method and roughly rolled, and then subjected to finish rolling. However, if the slab temperature after casting is high and the finishing temperature described below can be secured, rough slab heating is omitted. It doesn't matter. Moreover, when a thin slab is obtained by a known method such as strip casting, rough rolling may be omitted.
[0040]
Finish rolling: The hot rolled sheet as the base material of the high-strength steel sheet of the present invention has a uniform crystal structure based on ferrite and cementite so as not to form a band-like structure in the final product. If the finish rolling start temperature in hot rolling becomes too high, the crystal grains become coarse, which is not good. In order to avoid this, the finish rolling start temperature is set to 1050 ° C. or lower.
[0041]
The finish rolling finish temperature is an important condition in the steel of the present invention. If the finish rolling finish temperature is in a temperature range less than the Ar3 point, a ferrite band-like structure is formed before finish rolling and during finish rolling, which is not preferable. In order to avoid this, the finish rolling finish temperature is set to Ar3 point or higher.
[0042]
In order to promote the ferrite transformation after rolling, the more rolling distortion introduced into the austenite, the better. Therefore, the finish rolling finish temperature is preferably low. For this reason, finish rolling finish temperature shall be Ar3 point +100 degrees C or less.
[0043]
Auxiliary heating: It is desirable to satisfy the entry side temperature and the exit side temperature of the above finish rolling over the entire length of the hot-rolled coil. When the steel slab is long, the temperature of the steel slab decreases during rolling, and the finish rolling finish temperature may not be ensured in the latter half of hot rolling. Moreover, since the entrance temperature of finish rolling is limited low, the finish rolling finish temperature may not be ensured due to a temperature drop at the end of the steel slab width direction. In such a case, it is preferable to perform auxiliary heating on the finishing rolling entry side. The auxiliary heating method is not limited, but an electromagnetic induction heating method in which the heating amount can be easily controlled according to the temperature distribution of the steel slab on the finishing rolling entry side is preferable.
[0044]
Cooling conditions: Since the steel sheet of the present invention has a high Mn content, pearlite is easily formed in a band shape in the Mn segregation band formed during casting, and a band-like structure is easily formed in the product steel sheet.
[0045]
Since the pearlite transformation nose is 600 ° C. or lower, in order to avoid the formation of the band-like structure, the cooling rate after finishing rolling is 20 ° C./second or lower, and winding is performed at 600 ° C. or higher. By slowing the cooling rate before the pearlite transformation nose, the ferrite transformation is promoted, the pearlite volume fraction is decreased, and the formation of the pearlite band is suppressed. In particular, it is effective to gradually cool the temperature range of 800 to 700 ° C.
[0046]
If the coiling temperature is too low, the volume fraction of the bainite or martensite phase increases and subsequent cold rolling becomes difficult, so the coiling temperature is 600 ° C. or higher. In order to suppress a pearlite band more, it is desirable to wind up at 650 degreeC or more. If the winding temperature is excessively high, the pickling property is deteriorated. Therefore, in order to prevent this, the winding temperature is preferably set to 700 ° C. or less.
[0047]
Cold rolling: Cold rolling may be performed by a known method. However, when the rolling rate is excessively increased, a large number of fine voids are generated at the interface between ferrite and cementite in the steel sheet, and the bendability after annealing is deteriorated. If the rolling reduction is too small, the production efficiency is lowered. Therefore, the rolling rate during cold rolling is 50% or more and 70% or less.
[0048]
Annealing: The high-tensile steel sheet of the present invention has a tensile strength of 500 MPa or more and is a DP steel sheet that can be bent in any direction in the plate surface. In a uniform crystal structure containing 5 to 50% by volume.
[0049]
In order to realize this, the cold-rolled steel sheet is annealed in the two-phase region of ferrite + austenite in order to completely recrystallize the cold-rolled structure. The annealing time at that time is the time necessary for recrystallization, the lower limit is 30 seconds, and the upper limit is 90 seconds so that the grains do not become too coarse. Ac1 point and Ac3 point are known calculation formulas, for example, Ac1 point (° C.) = 723-10.7 Mn + 29.1Si, Ac3 point (° C.) = 910−203 √C + 44.7 Si (element symbol means mass%) It can be obtained by
[0050]
If the cold-rolled steel sheet is annealed for 10 to 90 seconds at a γ region temperature of Ac3 point or higher and Ac3 point + 80 ° C. or lower prior to annealing in the two-phase region, it is equiaxed without a band-like structure. Since the crystal structure can be obtained more stably, it is preferable for obtaining better bendability.
[0051]
In the present invention, in order to obtain a desired strength, martensite having a volume ratio of 5% or more is generated. Therefore, in order to avoid the formation of pearlite during the cooling from the annealing temperature, the temperature range from 750 ° C. to 550 ° C. is cooled at a cooling rate of 5 ° C./second or more. Since the temperature region of 550 ° C. or less is out of the precipitation nose of pearlite, a treatment such as a cooling rate of less than 3 ° C./second or a low temperature holding may be performed.
[0052]
What is necessary is just to manufacture by a well-known method except the above. For example, after annealing, temper rolling may be performed by a known method for the purpose of adjusting surface roughness, forcing flatness, and reducing elongation at yield point. Moreover, in order to manufacture a hot dip galvanized steel sheet, you may perform the said annealing process using a continuous hot dip plating line. An alloying heat treatment may be performed in order to obtain alloyed hot dip galvanizing.
[0053]
【Example】
Example 1
Steel having the chemical composition shown in Table 1 was melted in a laboratory to form a steel ingot having a thickness of 100 mm, a width of 300 mm, and a mass of 50 kg. The Ar3 point of these steels was around 780 ° C. The Ac1 point and the Ac3 point were determined by the known calculation formula.
[0054]
[Table 1]

In Table 1, steels H and I are comparative examples having a high C content, and others have chemical compositions that satisfy the conditions specified by the present invention. These steel ingots were charged into a heating furnace, held at 1200 ° C. for 60 minutes, then removed from the furnace, and roughly rolled to obtain a steel piece having a thickness of 30 mm.
[0055]
These steel slabs were subjected to hot finish rolling at a rolling start temperature of 1000 ° C. Some steel slabs were charged into a heating furnace before the start of finish rolling, and subjected to auxiliary heating to raise the temperature by about 50 ° C., so that the finish rolling start temperature was 1030 ° C. The number of finishing rolling passes was 6 in total, the thickness after finishing rolling was 4.0 mm, and the finishing rolling finishing temperature was variously changed between 850 ° C. and 750 ° C.
[0056]
After completion of hot finish rolling, the steel is cooled at various cooling rates in the range of 5 to 40 ° C./second, and charged in a heat treatment furnace at various temperatures of 700 to 600 ° C. in order to simulate the coiling temperature. , Held for 30 minutes and then cooled to room temperature at 20 ° C./hour. The obtained hot-rolled sheet was pickled using a hydrochloric acid solution to remove the scale, and then cold-rolled to 1.6 mm (1.2 mm for only two cases, 70% for the total rolling reduction) at a total rolling reduction of 60%. did.
[0057]
The obtained cold-rolled sheet was annealed under various conditions, and then cooled to room temperature at a cooling rate of 5 to 20 ° C./second. Although the annealing was mainly performed only with the two-phase region annealing, in the test numbers 3 and 9 described later, the two-phase region annealing was performed after annealing at the γ region temperature.
[0058]
Tensile properties: No. 5 as defined in JIS, in which the longitudinal direction of the obtained cold rolled steel sheet is the rolling direction (L), the width direction (T), or the direction (C) inclined by 45 ° from the rolling direction. A test piece was collected and subjected to a tensile test, and an average value in three directions was determined for tensile strength (TS) and total elongation (El).
[0059]
Bendability: From the obtained cold-rolled steel sheet, a bending test piece having the longitudinal direction as the above-mentioned three directions is collected and subjected to a close-contact bending test, and the outside of the bent portion is magnified with a magnifying glass and visually observed. Then, the occurrence of cracking or constriction on the surface was determined according to the following criteria, and a mark of “◯” was accepted. Constriction is obtained by cutting a cross section perpendicular to the bending axis of the bending portion of the test piece where necking tendency was observed, polishing and observing under a microscope, and the reduction rate of the plate thickness with respect to the plate thickness before the test is 20% or more. The thing was judged as “constricted”.
○: No cracking, no constriction,
Δ: No cracking, constriction,
X: Cracking occurred.
[0060]
Table 2 summarizes the manufacturing conditions and the characteristic value measurement results.
[0061]
[Table 2]

In Table 2, FFT is the hot rolling finish finish temperature, CR1 is the cooling rate after hot rolling, CT is the coiling temperature, RA1 is the γ region annealing temperature, RA2 is the two phase region annealing temperature, and CR2 is after the two phase region annealing. Cooling rate, TS means three-direction average of tensile strength, El means three-direction average value of total elongation.
[0062]
As shown in Table 2, in Test Nos. 1, 2, 3, 6, 8, 9, 11, 13 and 14, there was no cracking or constriction in the three-direction co-adhesion bending test, and good bendability was obtained. I had it. On the other hand, in the test numbers 4, 5, 10, 12, and 15 to 18, cracking or constriction occurred in any bending direction.
[0063]
(Example 2)
FIG. 1 is a perspective view of a channel-shaped component in which vertical wall portions are folded and closely bent to be overlapped. In FIG. 1, reference numeral B denotes a vertical wall bent portion, and the vertical wall portion is tightly bent at the A portion. In FIG. 1, a was 250 mm, b was 80 mm, and c was 100 mm.
[0064]
Within the plate surfaces of the steel plates of Test Nos. 1 and 18 of Example 1, the above-mentioned blank for the channel-shaped component whose length direction is the L direction or the T direction was cut out and formed into the channel-shaped component shown in FIG. The steel plate of test number 1 was processed satisfactorily without any cracks or constriction in the tightly bent portion (A) regardless of which direction the blank was collected. In the steel plate of test number 18, a crack was generated in the tightly bent portion (A) of a blank sampled with the longitudinal direction of the channel-shaped part set in the L direction.
[0065]
This means that in forming the channel-shaped part, if the length (a) exceeds the steel plate width, it is difficult to work with the steel plate of test number 18. Therefore, in order to obtain a part having the above-mentioned shape exceeding the width by the steel plate of test number 18, it does not depend on an inefficient method such as a method without contact bending or adding a plurality of parts in the longitudinal direction. I do not get.
[0066]
【The invention's effect】
Since the high-tensile steel plate of the present invention has excellent adhesion bendability in any direction in the plate surface, a material for bending can be obtained freely without being restricted by the direction in the plate surface. Therefore, a good molded product can be obtained by a method with a good yield without being restricted by the width of the material. Therefore, the high-tensile steel plate and the manufacturing method thereof according to the present invention greatly contribute to the expansion of application of the high-tensile steel plate. Industrial utility value is extremely high.
[Brief description of the drawings]
FIG. 1 is a perspective view of a channel-shaped component in which a vertical wall portion is bent and closely bent and overlapped. It is an external view of the model component which has a bending part of various directions.
[Explanation of symbols]

Claims (7)

In mass%, C: 0.05 to 0.10%, Si: 1.0% or less, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.01% or less, Steel having a chemical composition containing Al: 0.005 to 0.10%, N: 0.05% or less , the balance being Fe and inevitable impurities, performing rough rolling, and finishing rolling at 1050 ° C Start at the following, complete at Ar3 point or higher, Ar3 point + 100 ° C or lower, cool at a cooling rate of 20 ° C / second or lower, wind up at 600 ° C or higher to make a hot rolled sheet, and pickle this hot rolled sheet Cold rolling at a rolling reduction of 40 to 80%, and after performing a two-phase region annealing for 30 to 90 seconds in a two-phase region of Ac1 point or more and less than Ac3 point, up to 550 ° C up to 5 ° C / second A method for producing a high-tensile cold-rolled steel sheet excellent in bendability, characterized by cooling at the above cooling rate. 2. The chemical composition according to claim 1, further comprising 0.003 to 0.05% in total of one or more of the group consisting of Ti, Nb, and V in terms of mass%. A method for producing a high-tensile cold-rolled steel sheet with excellent bendability. Furthermore, as a chemical composition, Cr: 0.001-2.0% and / or Mo: 0.001-0.5% are contained by the mass%, The Claim 1 or 2 characterized by the above-mentioned. A method for producing a high-tensile cold-rolled steel sheet with excellent bendability. Furthermore, as a chemical composition, it is one of the group consisting of Ca: 0.0002 to 0.004%, Zr: 0.005 to 0.05%, and rare earth elements: 0.002 to 0.05% by mass%. Or 2 or more types are contained , The manufacturing method of the high tension cold-rolled steel plate excellent in the bendability in any one of Claims 1-3 characterized by the above-mentioned. Prior to the two-phase region annealing, the bending according to any one of claims 1 to 4, characterized in that γ region annealing is performed for 10 to 90 seconds at a γ region temperature of Ac3 point or higher and Ac3 point + 80 ° C or lower. A method for producing high-tensile cold-rolled steel sheets with excellent properties. The method for producing a high-tensile cold-rolled steel sheet with excellent bendability according to any one of claims 1 to 5 , wherein auxiliary heating is performed on a steel piece before finishing rolling. A method for producing a high-tensile hot-dip galvanized steel sheet, further comprising subjecting the high-tensile cold-rolled steel sheet obtained by the production method according to claim 1 to hot-dip plating.

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